Etherification processes are currently in great demand for making high octane compounds which are used as blending components in lead-free gasoline. These etherification processes will usually produce ethers by combination of an isoolefin with a monohydroxy alcohol. The etherification process can also be used as a means to produce pure isoolefins by cracking of the product ether. For instance, pure isobutylene can be obtained for the manufacture of polyisobutylenes and tert-butyl-phenol by cracking methyl tertiary butyl ether (MTBE). The production of MTBE has emerged as a predominant etherification process which uses C.sub.4 isoolefins as the feedstock. A detailed description of processes, including catalyst, processing conditions, and product recovery, for the production of MTBE from isobutylene and methanol are provided in U.S. Pat. Nos. 2,720,547 and 4,219,678 and in an article at page 35 of the Jun. 25, 1979 edition of Chemical and Engineering News. The preferred process is described in a paper presented at The American Institute of Chemical Engineers, 85th National Meeting on Jun. 4-8, 1978, by F. Obenaus et al. Other etherification processes of current interest are the production of tertiary amyl ether (TAME) by reacting C.sub.5 isoolefins with methanol, and the production of ethyl tertiary butyl ether (ETBE) by reacting C.sub.4 isoolefins with ethanol.
Due to the limited availability of olefins for etherification, it has become common practice to produce them by the dehydrogenation of isoparaffins and to pass the dehydrogenation effluent to an etherification process. Processes for producing olefins by the dehydrogenation of saturated hydrocarbons are well known. A typical dehydrogenation process mixes the feed hydrocarbons with hydrogen and heats the resulting admixture by indirect heat exchange with the effluent from the dehydrogenation zone. Following heating, the feed mixture passes through a heater to further increase the temperature of the feed components before it enters the dehydrogenation zone where it is contacted with the dehydrogenation catalyst. The catalyst zone may be operated with a fixed bed, a fluidized bed, or a movable bed of catalyst particles. After heat exchange with the feed, the dehydrogenation zone effluent passes to product separation facilities. The product separation facilities will typically produce a gas stream, made up primarily of hydrogen, a first product stream that includes the desired olefin products, and a second potential product stream comprising light hydrocarbons. The light hydrocarbon stream typically has fewer carbon atoms per molecule than the desired olefin product. Light hydrocarbons are generally removed from the product stream in order to reduce flow volume, operating pressures, and undesirable side reactions in downstream process units that receive the olefin product. A portion of the hydrogen stream is typically recycled to the dehydrogenation zone to provide hydrogen for the combined feedstream. The product stream usually contains uncoverted dehydrogenatable feed hydrocarbons in addition to the product olefin. These unconverted hydrocarbons may be withdrawn in separation facilities for recycle to the dehydrogenation zone or passed together with the product olefins to an etherification zone for conversion of the product olefins to ethers.
General representations of flow schemes where a dehydrogenation zone effluent passes to an etherification zone are shown in U.S. Pat. Nos. 4,118,425 and 4,465,870. More complete representations of a flow arrangement where the dehydrogenation zone effluent passes to an etherification zone are given in U.S. Pat. No. 4,329,516 and at page 91 of the October, 1980 edition of Hydrocarbon Processing. The latter two references depict the typical gas compression and separation steps that are used to remove hydrogen and light ends from the dehydrogenation zone effluent before it passes to the etherification zone. A typical effluent from an etherification zone includes an ether product, unreacted alcohol, and unreacted hydrocarbon and by-product ethers and alcohols. These effluent components enter separation facilities that yield the ether product, alcohol for recycling to the etherification zone, and hydrocarbons for further processing into dehydrogenation. This recycle stream of C.sub.4 or C.sub.5 isoparaffins, prior to recycling to the isomerization zone and the dehydrogenation zone, is usually treated to recover methanol and remove other oxygenates which are harmful to the isomerization and the dehydrogenation catalysts.
In the application where isomerization is used to produce more isoparaffin feed to the dehydrogenation unit U.S. Pat. No. 4,816,607, the recycle stream following the removal of oxygenates is combined with hydrogen and passed to a complete saturation process wherein any olefin and diolefins are saturated. The saturated stream is introduced to a fractionation zone along with additional C.sub.4 saturates. A side draw stream comprising normal C.sub.4 hydrocarbons is removed from the fractionation zone and passed over a catalyst in an isomerization reactor to convert the normal hydrocarbons to isoparaffins. The reactor effluent comprising isoparaffins is returned to the fractionation zone and a concentrated stream of isoparaffins is withdrawn from the top of the fraction zone and returned to the dehydrogenation zone.
The oxygenate compounds in the etherification zone effluent create problems such as catalyst deactivation or fouling in downstream processes that receive these unreacted hydrocarbons. For example where the unreacted hydrocarbons are recycled to a dehydrogenation zone, MTBE and tertiary butyl alcohol (TBA) may be present in the recycle stream. Oxygenate compounds present in the recycle stream can include etherification reactants such as alcohols. In particular the incomplete recovery of methanol from the etherification zone exacerbates the problem by increasing the oxygenate concentration in the recycle stream.
Oxygenates are often removed by adsorption processes. In typical operation of an adsorptive oxygenate removal unit the system uses two beds or multiples of two beds wherein one bed is operating in the adsorption mode and the other is operating in the regeneration mode.
In the adsorption art, 3-bed systems typically are used when the mass transfer zone for a particular separation is longer than one bed. The 3-bed configuration has two beds in series during the adsorption mode to allow the mass transfer zone to spill over from the lead bed into the second or trim bed in order to more completely load the lead bed to its equilibrium capacity. At the point of breakthrough from the trim bed, the unused adsorbent capacity remaining in the lead bed has normally reached less than 25% unused capacity. In a typical application for natural gas sweetening plants and natural gas dehydrators which exhibit long mass transfer zones, this lead/trim series configuration can add from 5-10% to the capacity of a single bed. A detailed discussion of adsorption systems including 3-bed configurations is described in an article at page 98 appearing in Hydrocarbon Processing, Volume 54, No. 2 and at page 86 appearing in Chemical Engineering, Jul. 9, 1973. In the adsorption of oxygenates from a light hydrocarbon mixture of iso and normal alkanes and alkenes, the mass transfer zone is relatively short and typically carried out in a single bed or part of a single bed (U.S. Pat. No. 4,814,517). Even though a 3-bed system is disclosed in U.S. Pat. No. 4,734,199 for a liquid phase process for the removal of methanol, each bed operates independently in the adsorption mode, although the beds are coupled during a liquid phase regeneration to provide conservation of the regenerant fluid.
Processes are sought which continuously produce an ultra pure product stream of unreacted hydrocarbons which are essentially-free of oxygenates. Although single bed systems have been proposed as described hereinabove, these single bed systems are not resilient to sudden variations in feed composition such as a spike of oxygenates resulting from an upset in upstream processing. In addition, a residual amount of spent regenerant remaining in an adsorbent bed following regeneration is often sufficient to contaminate the ultra pure product.